General Louis Blenker's Brigade was part of the Union Army during the First Battle of Bull Run (also known as the First Battle of Manassas) on July 21, 1861. Blenker's brigade was part of the Union Army's reserve force, which was held in reserve by General Irvin McDowell during the early stages of the battle.

However, as the battle turned against the Union Army, McDowell ordered Blenker's brigade to cover the retreat of the Union forces. Blenker's brigade was stationed at Centreville, Virginia, and was tasked with delaying the pursuing Confederate forces in order to give the rest of the Union Army time to retreat to safety.

Blenker's brigade fought a series of rearguard actions against the Confederate forces as they advanced, buying valuable time for the rest of the Union Army to escape. The brigade eventually fell back to the safety of Washington, D.C., along with the rest of the Union forces.

Despite the Union Army's defeat at Bull Run, Blenker's brigade was praised for its performance in covering the retreat. Blenker himself was later promoted to the rank of major general, and his brigade went on to serve in several more battles throughout the Civil War.